1. Field of the Invention
This invention relates to satellite communication systems and, in particular, to apparatus in such systems for reducing cross-polarization interference.
2. Description of the Prior Art
A crowding of the frequency spectrum in electromagnetic transmission systems has lead to an extremely limited availability of channels for radio and satellite communications. One technique for increasing the communicating capacity of a system is to utilize two orthogonal polarizations for a given frequency. In principle, if the polarization discrimination in a system is sufficiently precise, the same frequency-band can be shared by the various cross-polarization modes of transmission and the capacity of the system can be substantially increased.
However, certain problems arise when utilizing two orthogonal linearly polarized waves which must be overcome if satisfactory system operation is to be achieved. These problems center around the transmission channel and its effect on the multiple transmitted linearly polarized waves. In particular, after propagation through a transmission channel, the received polarizations are rarely "pure" (i.e., separated). This cross-polarization cross talk is caused by various sources such as rain, nonperfect antennas (especially when viewed off axis), ionospheric propagation, and the Faraday effect. The above-listed causes will essentially convert the linearly polarized waves into elliptically polarized waves having arbitrary orientations. Moreover, it should be noted that the effects of these causes of cross-polarization vary in time. Since the individual linearly polarized waves, after conversion to elliptically polarized waves, no longer have their original polarizations, they cannot be properly detected by the receivers of the respective system in which they are propagating without also detecting the cross-coupling signals from adjacent polarization signals, i.e., cross talk. Thus, when such a multiple polarization technique is employed, it is required that such unwanted cross talk induced between the polarizations during transmission and reception of information signals be held at or below an acceptable level in order to realize the full potential of a multiple polarization system. This reduction can be accomplished by means of a cross-polarization cancellation network.
Several RF correction networks have been described including networks by T. S. Chu (U.S. Pat. No. 3,728,643); Kannowade (U.S. Pat. No. 4,027,105); D. Difonzo of COMSAT, and those of Nippon and Marconi. However, only the Kannowade patent appears to disclose a system design which enables direct analog control of the polarization cancellation network; the others require a more general search optimization algorithm. The Kannowade patent utilizes a differential attenuator, a differential phase shifter, and two axially rotatable waveguide sections to cancel cross-polarization. However, this system essentially provides linearization of both polarizations at the same time, and this simultaneous linearization requires a relatively complicated control mechanism.